The creation of MOx surface species on pure silica MCM-48 using gas- and liquid-phase modifications with M-acetylacetonate complexes.

Abstract

Pure silica MCM-48 is prepared by a novel synthesis method, using the [C18H37N+(CH3)(2) - (CH2)(12) - N+(CH3)(2)C18H37].2Br(-) surfactant, abbreviated as GEMINI 18-12-18. The MCM-48, obtained after careful calcination, is a highly crystalline, mesoporous material, with the characteristics of the Ia3d cubic phase, a surface area exceeding 1000 m(2)/g and a narrow mesoporous pore size distribution (r = 1.4 nm; FWHH < 0.2 nm). This MCM support is grafted with VOx species, using a designed dispersion of VO(acac)(2) in a gas deposition reactor. In a first step, the complex is anchored to the support, in a subsequent step the adsorbed complex is thermolyzed to yield chemically bonded VOx surface species. The final material contains 1.7 mmol V/g (8.7 w% V), still has narrow pore size distribution and a surface area of 800 m(2)/g. It is observed that all silanols are consumed during the adsorption of the VO(acac)(2) complex to the MCM support. Therefore, the maximum achievable number of surface V-species is limited by the silanol number, and not by the geometrical surface, which has a higher capacity. After calcination of the adsorbed complex, the supported VOx species are present in a strictly tetrahedral configuration, but mainly as chains of linked tetrahedra and not as isolated species.